Britain had been worn and impoverished and battered and had lost an entire generation of men in the First World War. They were, perhaps, a giant by the time of WWII, but they were certainly not sleeping. The US is said to be a sleeping giant (by a Japanese admiral, no less) because it was isolationist and disinterested, largely, in world affairs, and while it has a massive capacity for economic output, that capacity was hardly being touched until the war.

Britain was the only world power at the start of the 2nd world war and was a military giant by any standards .
Germany and Japan were regional powers.. The British empire fought the war for two years in Europe Africa Asia minor and the far east.. add to the this the war of attrition fighting the war at sea put enormous strain on the British Empire, the war brought huge losses and no gains at first... but gradually bit by bit the tide was turned in north Africa with the defeat of the German Africa corps At the start of the war The US was an isolationsit country that didnt want to get involved in the war.. When japan made the mistake of bombing pearl harbour , it woke a sleeping giant (usa) bringing them into the war.

Napoleon told the world to beware of the sleeping giant it was China that is history not fake American bragging again and you awoke the sleeping giant for cheap crap it will soon be the boss learn how to bow they are coming

No, Britain was defending itself against the Luftwaffe before the invasion of Poland. Britain was preparing for war. The U.S. was not prepared for war, it is thousands of miles from Europe, and its factories were not re-tooled for weapons of war until after Pearl Harbor, thats what is meant by a sleeping giant, in reference to the Pearl Harbor attack. But Britain was well aware of the danger of Germany in 1933 after Hitler was appointed.